Gasoline, alcohol such as ethanol and methanol, and alcohol mixed fuel of gasoline and alcohol are used as fuel for an internal combustion engine. In a system including such an engine, as shown in JP-1-113528A, an alcohol concentration in the fuel is detected by a fuel sensor. An ignition timing and an air-fuel ratio are determined based on the alcohol concentration, whereby the ignition timing and the air-fuel ratio are controlled preferably.
Generally, in an internal combustion engine, a combustion efficiency becomes best at minimum advance for the best torque (MBT) that is immediately before a knocking occurs. As the alcohol concentration and the octane number become high, a knock limit of the ignition timing is advanced. In a system where the ignition timing is established according to the alcohol concentration, an output torque of the engine can be increased while the knocking is prevented by advancing the ignition timing as the alcohol concentration becomes high. In such a ignition timing control, in a case that a low-alcohol-concentration fuel is used, the ignition timing is retarded relative to a case of a high-alcohol-concentration fuel, which may reduce the output torque and deteriorate a drivability of the engine. The fuel of which alcohol concentration is low is referred to as low-alcohol-concentration fuel and the fuel of which alcohol concentration is high is referred to as high-alcohol-concentration fuel.